In general, an integrated circuit refers to an electrical circuit contained on a single monolithic chip containing active and passive circuit elements. As should be well understood in this art, integrated circuits are fabricated by diffusing and depositing successive layers of various materials in a preselected pattern on a substrate. The materials can include semiconductive materials such as silicon, conductive materials such as metals, and low dielectric materials such as silicon dioxide. The semiconductive materials contained in integrated circuit chips are used to form such conventional circuit elements as resistors, capacitors, diodes and transistors.
Integrated circuits are used in great quantities in electronic devices such as digital computers because of their small size, low power consumption and high reliability. The complexity of integrated circuits ranges from simple logic gates and memory units to large arrays capable of complete video, audio and print data processing. As the semiconductor industry strives to meet technological demands for faster and more efficient circuits, integrated circuit chips and assemblies are created with reduced dimensions, higher operating speeds and reduced energy requirements. As integrated circuit signal speeds increase, timing errors and pulse width deviations within such signals may constitute a greater portion of a signal period that the signal itself.
Timing fluctuations in integrated circuits are generally referred to as “jitter”. Jitter can be broadly defined in certain interpretations as the variation of a signal edge from its ideal position in time, and can be an important performance measure for integrated circuit signals, including serial links and clock signals. For serial link qualification, jitter is decomposed into its various components, which are generally divided into two types, deterministic and random. The impact of each jitter component on bit error rate (BER) performance is different.
While random jitter is unbounded and is due to sources that can only be characterized statistically, deterministic jitter is bounded and may be correlated to known sources such as supply voltage fluctuations, control-system instability, temperature variation, noise and the like. Deterministic jitter has two main contributing portions, namely periodic jitter (PJ) and data-dependent jitter (DDJ). DDJ behaves as a high-frequency jitter that is strongly correlated to a data stream's bit pattern. The main sources of DDJ in a signal are related to inter-symbol interference (ISI) and signal reflections.
ISI may typically be the result of bandwidth limitations of a transmission channel, which causes single bit information to spread into adjacent transmitted data bits. In some instances, the impact of ISI on DDJ may be affected by slew rate and/or phase distortions. Slew rate concerns the rate of change of voltage levels in a signal. Binary signals that include signal changes from a “0” bit defined by a first predefined voltage level and a “1” bit defined by a second predefined voltage level ideally have an infinite slew rate. However, bandwidth limitations of existing transmission channels result in a finite transition rate between first and second voltage levels representative of adjacent “0” bits and “1” bits, resulting in the leaking of bit information into adjacent data bits. With regard to phase distortions or group delay variations, it is noted that some channels have very fast changing phase characteristics within specific frequency ranges (often close to the pass-band to stop-band). In such cases, slight variations of data bit rate due to signal transition density, or channel parameters, can result in significant variations in bit transition edge delay.
Referring still to the different possible sources of DDJ, it should be appreciated that transmission line reflections may also contribute to DDJ in a transmitted signal. Reflection may occur on channels comprised of transmission lines with mismatched termination impedances. If mismatch exists in both ends of a transmission line, a receiver will receive a delayed and attenuated version of the transmitted signal in addition to the transmitted signal. The amount of delay and attenuation depends on the transmission line characteristics and the amount of termination mismatches.
In practical transmission channels, the primary sources of DDJ often correspond to ISI and related effects of slew rate. However, in situations where the channel consists of multiple transmission lines (e.g., including but not limited to multiple printed circuit board traces, relays, connectors, intermediate terminations, etc.), reflections and phase-distortion ISI may also become significant. Test fixtures and connectors associated with automated testing equipment may also contribute to possible signal reflections and/or phase distortion ISI.
The measurement and determination of signal jitter, including the various components thereof, is imperative in characterizing the performance of integrated circuits, especially in the production and testing stages of integrated circuit manufacturing. Various devices, including time interval analyzers, counter-based measurement devices and oscilloscopes, have been developed to measure various signal timing deviations, including jitter.
An example of a time interval analyzer that may be employed to measure high frequency circuit signals and determine various aspects of signal timing deviations is disclosed in U.S. Pat. No. 6,091,671 (Kattan), which is assigned to the present applicants' assignee, Guide Technology, Inc. The time interval analyzer disclosed in Kattan measures jitter, including total cycle-to-cycle jitter, by determining deviations between one or more of the amplitude, phase, and/or pulse width of real signal pulses and ideal signal pulses.
Other examples of time measurement devices that could be configured to measure signal timing variations are disclosed in U.S. Pat. No. 6,194,925 (Kimsal et al.) and U.S. Pat. No. 4,757,452 (Scott et al.) Kimsal et al. discloses a time interval measurement system in which a voltage differential across a hold capacitor generated between events occurring in an input signal determines the time interval between events. Scott et al. provides a system for measuring timing jitter of a tributary data stream that has been multiplexed into a higher-rate multiplex stream using pulse stuffing techniques. Scott et al. is an event counter based system that does not directly measure time intervals but determines their frequency by maintaining a continuous count of the number of pulses occurring within a signal. Still further, U.S. Pat. No. 4,908,784 (Box et al.) discloses a measurement apparatus configured to measure the time interval between two events (start and stop) through counters.
As referenced above, several devices exist for measuring signal properties, including timing variations such as total signal jitter. However, specific types of signal analysis must be applied to signal measurements in order to extract the different components of a signal (e.g., jitter signal) so that the source of jitter can be more easily characterized. U.S. Pat. No. 6,356,850 (Wilstrup et al.) discloses features and steps for separating the components of a jitter signal, including the random and periodic components of the jitter signal. U.S. Pat. No. 6,298,315 (Li et al.) discloses features and steps for separating and analyzing the random and deterministic components of a distribution using tail-fitting steps and estimation of associated statistical confidence levels.
Although the above examples and others exist for measuring and analyzing various aspects of signal jitter, no one design exists that encompasses all features and aspects of the present invention.
All the aforementioned patents are incorporated herein by reference for all purposes.